1. Field of the Invention
The present invention relates to an implantable heart monitoring device, with which it is possible to monitor the heart condition. The invention also concerns a corresponding system and method.
2. Description of the Prior Art
Several different devices for monitoring the performance of a heart are known. Often these devices are also able to deliver stimulation pulses to the heart. The devices are often able to sense the electrical activity in the heart. It is also known to determine an impedance value measured between different electrodes positioned in or at the heart. It is also known to sense other physiological parameters, such as pressure, oxygen level etc.
U.S. Pat. No. 5,154,171 describes the sensing of impedance values in order to control the pacing rate.
U.S. Pat. No. 6,070,100 describes that electrodes may be positioned in or at both the left and the right atria as well as in or at the left and the right ventricles. The document describes the possibility of sensing the impedance between different electrodes. The sensed impedance values may be used to improve the cardiac output.
United States Patent Application Publication No. 2001/0012953 describes bi-ventricular pacing. An impedance may be measured between electrodes on the right and the left sides of the heart. The variation of the impedance with time is detected. The detected impedance variation may be used in order to synchronise the contraction of the ventricles.
United States Patent Application No. 2001/0021864 describes different manners of using the proximal and distal electrodes of different leads in order to inject a current and to measure an impedance. The measured impedance value may be used in order to maximise the cardiac flow.
U.S. Pat. No. 6,314,323 describes a heart stimulator in which the cardiac output is determined by measuring the systolic pressure.
It is also known to communicate with an implanted heart monitoring/stimulating device in a wireless manner, i.e. with the use of so-called telemetry. This can be done by inductive communication or via radio waves. With the help of telemetry it is thus for example possible to obtain information about the status of an implanted device. It is also known to input new information into the device with the help of such telemetry.
For different severe cardiac conditions it may be important to monitor the status of the heart, for instance in order to follow the progress of a heart disease, for example in order to be able to carry out a suitable treatment of the patient. One such heart condition is congestive heart failure (CHF). This is a condition in which the heart's function as a pump to deliver oxygen rich blood to the body is inadequate to meet the body's needs.
Another concept used in this field is pre-load. Pre-load can be defined as the initial stretching of the cardiac myocytes prior to contraction. The concept of pre-load can be applied to either the ventricles or atria. Regardless of the chamber, the pre-load is related to the chamber volume just prior to contraction.